Thin film magnetoresistive sensors or heads have been used in magnetic data storage devices for several years. The fundamental principles of magnetoresistance including anisotropic magnetoresistance (AMR), giant magnetoresistance (GMR) and spin tunneling have been well-known in the art for some time. Magnetic readback sensor designs have been build using these principles and other effects to produce devices capable of reading high density data. In particular, three general types of magnetic read heads or magnetic readback sensors have been developed: the anisotropic magnetoresistive (AMR) sensor, the giant magnetoresistive (GMR) sensor or GMR spin valve and the tunnel valve sensor. The construction of these sensors is discussed in the literature, e.g., in U.S. Pat. No. 5,159,513 or U.S. Pat. No. 5,206,590.
FIG. 1A shows a typical GMR readback sensor according to the prior art. The sensor has a high coercivity ferromagnetic pinned layer 4 with a fixed magnetic moment pointing unto the page and a low coercivity ferromagnetic free layer 5 with a movable magnetic moment, which can rotate from pointing into the page to pointing out of the page. The direction of the magnetic moment of the pinned layer 4 is fixed by exchange coupling with an antiferromagnetic layer 3. Pinned layer 4 and free layer 5 are separated by a thin film of copper 7 or other metal with a relatively long electron mean free path. Hard bias layers 6 provide a biasing magnetic field to the free layer 5.
This construction of the GMR sensor enables the magnetic moment of free layer 5 to rotate rapidly and easily in an externally applied magnetic field. Typically, such rotation of the magnetic moment of free layer 5 occurs during operation in response to an external magnetic field encoding corresponding data, e.g., in a track of a magnetic disk. In accordance with the well-known principles of magnetoresistance, rotating the magnetization alters the resistance of the GMR sensor to a current i passing between a first electrical contact 8A and a second electrical contact 8B by a certain value AR. (In general, the larger the value of .DELTA.R in relationship to total resistance R, i.e., the larger .DELTA.R/R the better the sensor.) This change in resistance due to rotation of the magnetization of free layer 5 can thus be electronically sensed and used in practical applications such as reading of magnetic data.
An important concern in the design of the sensor of FIG. 1A is the longitudinal bias of the free layer 5. The longitudinal direction is the direction in the plane of the air bearing surface and parallel to the layers of the sensor, i.e., from left to right in FIG. 1A as indicated by arrow L. The free layer must be biased by hard bias layers 6 so that the free layer has only a single magnetic domain. When no biasing is present, the magnetic moments in free layer 5 tend to establish a multi-domain state, as is well-known. When free layer 5 is allowed to have more than one magnetic domain, then it experiences Barkhausen jumps and other domain reorientation phenomena during magnetic reversal as when responding to external magnetic fields encoding data in a magnetic recording disk. This problem is also known in the art and is highly undesirable as it produces noise and worsens the signal-to-noise ratio (SNR) of the sensor.
In order to provide the longitudinal biasing field and prevent Barkhausen noise several schemes have been employed. One uses hard bias layers 6 that have a high coercivity. For more details on longitudinal biasing using a hard biasing ferromagnetic layer the reader is referred to U.S. Pat. No. 5,729,410 to Fontana, Jr. et al.
Another longitudinal biasing scheme, sometimes also referred to as a continuous spacer design scheme, is illustrated in FIG. 1B. This figure uses the same reference numerals as FIG. 1A to designate corresponding parts. The order of layers in sensor of FIG. 1B is opposite from that in FIG. 1A and an additional buffer layer 1 and cap layer 2 are provided. In this scheme a single magnetic layer 9A, e.g., a Ni--Fe layer, is fixed or pinned by an antiferromagnet 9n in contact with magnetic layer 9A and both are located adjacent free layer 5. Magnetic layer 9A provides a magnetic moment indicated by the arrow adjacent to free layer 5 for longitudinal biasing of free layer 5. For more details on this longitudinal biasing scheme the reader is referred to U.S. Pat. No. 5,528,440 to Fontana et al.
For proper biasing, the longitudinal bias layers should have a remnant magnetization several times the thickness product (M.sub.r T.sub.bias) that is proportional to the M.sub.r T.sub.free of the free layer. A typical value for the hard bias M.sub.r T.sub.bias product is 1.7 times the free layer M.sub.r T.sub.free product.
Developers of data storage devices are constantly striving to reduce the dimensions of readback sensors so that the data density of magnetic data storage products is increased. In order to increase the data density, the thickness of the free layer must be decreased. For a data density of about 3 Gb/in.sup.2, the free layer should have a magnetic moment equivalent to that of about 70 Angstroms of Ni.sub.80 Fe.sub.20 alloy; for a data density of about 40 Gb/in.sup.2, the free layer should have a thickness equivalent to about 45 Angstroms of Ni.sub.80 Fe.sub.20 alloy. For typical materials used the physical thickness of the free layer will drop from about 70 Angstroms at 3 Gbit/in.sup.2 to as thin as 30 Angstroms at 40 Gbit/in.sup.2.
Reducing the thickness (and therefore M.sub.r T.sub.free) of the free layer requires a corresponding reduction in the M.sub.r T.sub.bias of the hard bias layers. Specifically, the proportionality constant between the free layer M.sub.r T.sub.free and hard bias M.sub.r T.sub.bias (i.e. 1.7) should be roughly preserved, although the exact value depends on specific geometric considerations. However, problems are encountered when one attempts to reduce the M.sub.r T.sub.bias value of the hard bias layers. Under these conditions, the reduction in coercivity of CoPt alloys typically used in hard bias layers is a well-known problem.
The M.sub.r T.sub.bias value of the hard bias layer can be reduced by decreasing its thickness. However, this is undesirable because it reduces the coercivity of the hard bias layer, making it susceptible to external fields and its own demagnetizing fields and thus making it less effective at suppressing Barkhausen noise. Further, a hard bias layer with reduced thickness may be unstable and magnetically decays over time. This could result in a reduced lifetime of the readback sensor. The magnetic stability of hard bias layers for hard bias could be increased by increasing the magnetic anisotropy of the hard bias layer material, but this has proved difficult with conventional CoPt based materials.
Referring back to Fig. 1B, schemes that employ longitudinal bias using antiferromagnet 9B in contact with a single magnetic layer 9A (continuous spacer design) need that antiferromagnet 9B to stabilize bias magnetic layer 9A along the longitudinal direction. Since the longitudinal direction is not in the direction of pinned layer 4, which uses antiferromagnet 3 a difficulty arises in setting antiferromagnets 3 and 9B simultaneously. Thus, a scheme which does not use an antiferromagnet in the longitudinal bias is desirable. However, if an antiferromagnet is used, it is desirable to increase the exchange field between the antiferromagnet and longitudinal bias layer to increase the magnetic stability of the longitudinal bias so that it is not disturbed by external fields or its own demagnetizing field.
Finally, there exists a need in the art of micromagnetic sensors for a longitudinal biasing structure that is capable of providing longitudinal biasing of extremely thin ferromagnetic films. Such a longitudinal biasing structure would facilitate the use of magnetic readback sensors in data storage devices having extremely high data densities.